A. Field of the Invention
The present invention relates to a new and improved method of preparing tetraselenofulvalenes and to certain highly electrically conductive organic compounds made by coupling the tetraselenofulvalenes with an electron acceptor.
B. Related Application
Copending U.S. application Ser. No. 440,221 filed by D. O. Cowan and A. N. Bloch now abandoned discloses certain highly electrically conductive organic salts obtained by reacting together an electron donor, e.g. tetrathiofulvalene (TTF) and an electron acceptor such as tetracyano-p-quinodimethane (TCNQ). The salt obtained, using TTF and TCNQ as reactants for purposes of illustration, is called tetrathiofulvalenium-tetracyano-p-quinodimethanide or, more conveniently, TTF.sup.+ -TCNQ.sup.-. This product is comprised of separate homologous columnar stacks of cations and anions, TTF.sup.+ being the cation and TCNQ.sup.-, the anion. The stacks of radical cations and radical anions in the crystal structure of the product provide a pseudo one-dimensional network for electrical conduction which is an important characteristic of the compounds described in Ser. No. 440,221. Other characteristics of these products include the following:
(a) Both the cation and anion are "open shell", i.e., both have a density of unpaired electrons, which may be fractional. This is an important feature because, if either cation or anion (or both) are not "open shell", lower electrical conductivities result.
(b) Both the cation and anion have a high degree of symmetry. A lack of symmetry can result in electron localization and lower electrical conductivities.
(c) The cation and anion are polarizable and thus reduce electron-electron repulsions.
(d) The compounds from which the salt is made must have at least two "easily available" oxidation states, the transfer of electrons requiring as little energy as possible. Thus, the reduction potential of the anion and the oxidation potential of the cation are low and are as close together as possible, the anion being a good electron acceptor and the cation being a good electron donor.
(e) The molecular charge densities are inhomogeneous and thus facilitate stacking.
The highly electrically conductive salts of Ser. No. 440,221 may be generically described as compounds of the formula (D.sup.+n).sub.x (A.sup.-m).sub.y wherein D.sup.+n is a radical cation and A.sup.-m is a radical anion, .sup.+n is the formal positive charge on the cation D, x is the number of cation species present, .sup.-m is the total negative charge on the anion A, and y is the number of anion species present, the absolute values of the respective products (+n) (x) and (-m) (y) being equal, and wherein D and A are highly symmetric and highly polarizable, and, respectively, have oxidation potentials and reduction potentials which are low and relatively close in absolute value, D and A being present in segregated columnar stacks in the compound, thereby making the compound essentially one-dimensional in nature.
The cation D is more specifically defined in Ser. No. 440,221 as being formed from a member of the group consisting of compounds of the formula ##STR1## wherein
X.sub.1 =X.sub.2 =X.sub.3 =X.sub.4 =S, Se, or Te;
X.sub.1 =X.sub.4 =S, X.sub.2 =X.sub.3 =Se or Te;
X.sub.1 =X.sub.3 =S, X.sub.2 =X.sub.4 =Se or Te;
X.sub.1 =X.sub.2 =S, X.sub.3 =X.sub.4 =Se or Te;
R.sub.1 =R.sub.2 =R.sub.3 =R.sub.4 =H;
R.sub.1 =R.sub.2 =R.sub.3 =R.sub.4 =CH.sub.3 ;
R.sub.1 =R.sub.4 =H, R.sub.2 =R.sub.3 =CH.sub.3 ;
R.sub.1 =R.sub.3 =H, R.sub.2 =R.sub.4 =CH.sub.3 ;
R.sub.1 =R.sub.2 =H, R.sub.3, R.sub.4, =CH.sub.3 ;
R.sub.1 =R.sub.2 =R.sub.3 =R.sub.4 =CF.sub.3 ;
R.sub.1 =R.sub.2 =R.sub.3 =R.sub.4 =--CN;
R.sub.1 =R.sub.2 =R.sub.3 =R.sub.4 =--COOH;
R.sub.1 =R.sub.4 =H; R.sub.2 =R.sub.3 =.phi.; and,
R.sub.1 =R.sub.3 =H; R.sub.2 =R.sub.4 =.phi..
TTF is a compound, or electron donor, according to Formula I above, wherein X.sub.1 -X.sub.4 are each S and R.sub.1 -R.sub.4 are each hydrogen. Thus, TTF may be shown structurally as follows: ##STR2##
The anion A may be formed from a variety of different electron acceptors. Preferably, however, the anion is derived from TCNQ which may be structurally represented as follows: ##STR3##
The disclosure of Ser. No. 440,221 is incorporated herein by reference.